1. Field of the Invention
The present invention is directed to an AC-to-DC converter for providing a step-down output DC voltage, and more particularly to such a converter including a rectifier and a chopper.
2. Description of the Prior Art
An AC-to-DC converter has been widely utilized in the art, for example, in an electronic ballast where it is combined with an inverter to drive a discharge lamp from an AC voltage source such as an AC mains. In this instance, the AC-to-DC converter is required to supply a smoothed output DC voltage to the inverter in order to operate the discharge lamp efficiently free from flickering. Further, in view of that the output DC voltage should be as smooth as possible in order to eliminate higher frequency harmonics in an input AC current to the converter for reducing corresponding noises as well as to improve the power factor of the converter, the converter is highly desired to include a chopper in addition to a rectifier for providing a smoothed output DC voltage. Consequently, when the converter is designed to provide a step-down DC voltage, it is readily possible to include a step-down chopper or inverted chopper in the converter. However, the choppers of theses types are known to inherently suffer from reduced power factor and are likely to cause undesired higher frequency harmonics which induce noises in other electrical devices powered by the same AC voltage source, in contrast to a step-up chopper which is known to exhibit improved power factor. In order to avoid this problem, it has been proposed to use a step-up chopper in front of the inverted chopper in order to compensate for the reduced power factor of the latter, while assuring to output the step-down DC voltage, as disclosed in Japanese Patent Early Publication (KOKAI) No. 2-237461. As represented in FIG. 1 of the attached drawings, this converter comprises a full-wave rectifier 110 providing a rectified DC voltage from an AC voltage source 100, the step-up chopper 120 and the inverted chopper 150. The step-up chopper 120 includes a first inductor 122 connected in series with a first FET 121 across the rectifier 110, and a first smoothing capacitor 125 connected in series with a first blocking diode 123 across the FET 121. The FET 121 is controlled by a first driver 126 to turn on and off at a high frequency so as to store an energy in the inductor 122 from the rectifier 110 when FET 121 is conductive. Upon subsequent turn-off of FET 121, the inductor 122 releases the energy so that the smoothing capacitor 125 is supplied through the diode 123 with the energy from the inductor 122 as well as from the rectifier 110 to develop a step-up DC voltage. The inverted chopper 150 comprises a second FET 151, a second inductor 152 and a second smoothing capacitor 155 which are connected in series across the first smoothing capacitor 125 with a second diode 153 connected in series with the FET 151 across the first smoothing capacitor 125. The FET 151 is controlled by a second driver 156 to turn on and off at a high frequency so as to store an energy into the inductor 152 when FET 151 is conductive. Upon subsequent turn-off of FET 151, the inductor 152 releases the energy which is alone supplied through the diode 153 into the smoothing capacitor 155 to develop thereat the step-down output DC voltage to an inverter 170 for driving a load. Although the converter can successfully assure improved power factor by the use of the step-up chopper in front of the inverted chopper, there still remains a problem due to the use of the inverted chopper in that all the energy supplied to smoothing capacitor 155 is once stored in the inductor 152, which eventually lowers overall circuit efficiency. Further, the use of the step-up chopper 120 requires the first smoothing capacitor 125 of a high capacitance in order to temporarily store the step-up voltage, in addition to the second smoothing capacitor 155 which may be of a low capacitance for storing the step-down output DC voltage, and also requires the two inductors in the converter, thus causing duplication of expensive components.
Another prior art converter is disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 63-37584 to use a charge-pump circuit instead of the chopper, as represented in FIG. 2 of the attached drawings. The charge-pump circuit 250 is connected to the output of a like full-wave rectifier 210 to receive the voltage therefrom and store the voltage in a series combination of capacitors and 252. A diode network of diodes 254, 256, 258 is connected to the capacitors 251 and 252 so as to form parallel paths through which the individual capacitors 251 and 252 are discharged respectively in parallel relation to supply a step-down DC voltage to an inverter 270. In this manner, this charge-pump circuit 250 can provide the step-down output DC voltage while improving the power factor. However, it is still unsatisfactory in maximizing the power factor and is therefore difficult to eliminate the higher frequency harmonics in an input AC current to the rectifier.